With continuous development of wireless communications system technologies, to implement communication, a D2D communication mode may be used in addition to a conventional cellular communication mode. In the cellular mode, user equipment (English: User Equipment, UE for short) communicates with another UE by using a base station. However, in the D2D communication mode, UE directly communicates with another UE. By means of D2D communication, not only a radio spectrum resource is saved, but also pressure on a core network is reduced. Therefore, a D2D technology becomes an important technology that is currently being standardized in the 3rd Generation Partnership Project (English: 3rd Generation Partnership Project, 3GPP for short) standard and that can be compatible with the 5th Generation (English: 5rd Generation, 5G for short).
Currently, enhanced D2D (English: Enhanced D2D, eD2D for short) is a topic being studied in the 3GPP Long Term Evolution (English: Long Term Evolution, LTE for short) system release.13 (English: Release.13, Rel.13 for short). In this topic, UE-to-network relays (English: UE-to-Network Relays) are an important research orientation. The UE-to-network relays are classified into two scenarios: communication (English: communication) and discovery (English: discovery). For the two scenarios, a specific communication process, resource allocation, data transmission, and the like need to be designed and studied correspondingly.
Currently, how to resolve a resource allocation problem of a link from UE out of network coverage (English: out of coverage) to UE in the network coverage (English: in coverage) becomes an urgent problem to be resolved.